1. Field of the Invention
This invention relates to the area of non-destructive inspection technology known as magneto-optic/eddy current imaging.
2. Discussion of Related Art
Conventional magneto-optic/eddy current imaging technology has been used to generate real-time images of defects, such as fatigue, cracks and corrosion, in metals, and cracks, delamination and other defects in non-metallic structures. Instruments which utilize the Faraday magneto-optic effect, first discovered in 1845, operate by employing a material which rotates the plane of polarization of polarized light passing through the material as a function of an applied or developed normal magnetic field.
As shown in FIG. 1, a typical magneto-optic effect sensor employs a magneto-optic crystal, such as a garnet-iron crystal 1, coated with a dielectric reflective layer 2 which causes light passing through the crystal from a source 3 and polarizer 4 to reflect back through the crystal to a polarization detector 5, as shown in FIG. 1. On each pass of polarized light through the crystal, the initial polarization is rotated in a direction determined by an the applied magnetic field B. Although the reflective coating 2 is not essential, by utilizing a double pass configuration, the rotational sensitivity of the sensor is increased by a factor of two.
Such a sensor can be used for non-destructive structural inspections by applying an alternating magnetic field to the structure being tested or inspected test. The alternating magnetic field induces a uniform flow of eddy currents in the structure. Pursuant to Lenz's law, the eddy currents in turn create weak secondary magnetic fields which oppose the applied field and are therefore normal to the eddy currents and to the plane of the sensor. The low intensity secondary magnetic fields cause a local rotation of the plane of polarization of the light passing through the sensor. The local rotation of the plane of polarization creates a spatial image on the sensor, which is latent until light passing through the sensor is intercepted by the polarization detector or analyzer 4 which converts variations in polarization to intensity modulation. Any cracks, surfaces, or sub-surface anomalies in the test sample or structure being inspected will disrupt the flow of eddy currents and therefore vary the secondary magnetic fields, the variations being detected by the magneto-optic sensor in order to determine the nature and extent of the anomaly.
A disadvantage of conventional systems using the type of sensor shown in FIG. 1 is that when an induced eddy current is parallel to a crack or other anomaly, the level of disturbance available to create the perpendicular secondary magnetic field needed to rotate the local polarization in the magneto-optic crystal, and thereby detect the crack, will be negligible, increasing susceptibility of the image formed by the sensor to background interference effects. As a result, conventional magneto-optic measuring instruments must be manually rotated in order to ensure that anomalies in all directions are detected. This slows the inspection process, puts a limitation on miniaturization and the use of automized pattern detection techniques, and makes remote un-manned inspections using magneto-optic technology impossible.